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Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
Both excess and deficiency of thyroxine can cause disorders. Hyperthyroidism (an example is Graves' disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.
When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones.
Thyroxine, also known as T 4, is a hormone produced by the thyroid gland. It is the primary form of thyroid hormone found in the blood and acts as a prohormone of the more active thyroid hormone, triiodothyronine (T 3 ). [ 1 ]
Subclinical hyperthyroidism is a milder form of hyperthyroidism characterized by low or undetectable serum TSH level, but with a normal serum free thyroxine level. [32] Although the evidence for doing so is not definitive, treatment of elderly persons having subclinical hyperthyroidism could reduce the number of cases of atrial fibrillation. [33]
Thyroid's secretory capacity (G T, also referred to as thyroid's incretory capacity, maximum thyroid hormone output, T4 output or, if calculated from serum levels of thyrotropin and thyroxine, as SPINA-GT [a]) is the maximum stimulated amount of thyroxine that the thyroid can produce in a given time-unit (e.g. one second).
Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T 4), and then triiodothyronine (T 3) which stimulates the metabolism of almost every tissue in the body. [1]
Normal thyroid levels are also seen, and occasionally also hypothyroidism, which may assist in causing goiter (though it is not the cause of the Graves' disease). Hyperthyroidism in Graves' disease is confirmed, as with any other cause of hyperthyroidism, by measuring elevated blood levels of free (unbound) T3 and T4. [citation needed]